In general, methods for measuring the concentration of materials in solutions are classified into off-line method and on-line method, according to the nature of measurement or measuring device.
In the measurement of cell concectration during a fermentation process, the off-line method includes a dry weight measurement and an absorbance measurement where a spectrophotometer is employed to determine the absorbance of diluted medium. In the latter method, dilution is inevitable since the linearity between the absorbance and cell concentration is rather limited, grounded on the usage of a conventional quartz-made 1 cm cuvette or a test tube of 1 cm diameter, which, in turn, restricts the available measurement range to a very low concentration. To overcome this limitation of narrow measuring range, a device which can measure high concentration by regulating light path length has been suggested in the art (see: Fugita, T., Nunomura, K., Appl. Microbiol., 16(2):212-216(1968)). However, the device has been proven to be rather limited to measuring of microorganism by off-line method. The off-line method has further shortcomings as follows: when sampling is carried out in a fermenter, the fermenter may be contaiminated with saprophytes; errors may be generated in frequent handling and dilution of the sample; and, real-time assay is impossible. Therefore, many concerns and works are devoted to the on-line measurement which does not need to flow out the culture from a fermentor like sampling or dilution.
In one-line measurement, since there is no outflow of sample, measuring of the dissolved substances and solid materials in a reactor under a chemical or biological process can be performed to guarantee easy control and suitable operation. Until now, as one of the on-line measuring devices without outflow and dilution of the reactants, an electrode equipped measuring device fitted to a reactor has been proposed in the art. However, this device has also revealed some drawbacks of expensiveness, instability, and errors caused by the reactor condition.
Therefore, there are strong reasons for developing a novel on-line measuring device with stability and accuracy which can simply determine the concentrations of dissolved substances, solid materials, and products in many biological or chemical processes.